The present invention relates to an improved method of forming a soot preform from which optical waveguide fibers can be made.
Certain glass making processes, particularly vapor deposition processes, have been commonly employed in the formation of optical waveguide blanks. In one such process, referred to as the flame hydrolysis or outside vapor deposition process, a plurality of constituents in vapor form are entrained in a gaseous medium in predetermined amounts and thereafter are oxidized in a flame to form particulate matter called soot. A first soot coating of uniform or radially varying composition is applied to the surface of a rotating cylindrical mandrel or starting member. After the first coating of soot is deposited to form the core glass, the composition of the soot is changed to form a cladding glass coating. Heretofore, the mandrel was usually removed, and the soot preform was sintered to form a consolidated, clear glass draw blank having an aperture therethrough. Acid was flowed through the aperture to etch defects from the aperture forming surface. The resultant tubular draw blank was heated to a temperature at which the material had a low enough viscosity for drawing and was drawn to collapse the hole therein and form a fiber having the desired dimensions. Various methods employing such a flame hydrolysis technique for forming glass optical waveguide fibers are taught in U.S. Pat. Nos. Re. 28,029; 3,711,262; 3,737,293; 3,823,995 and 3,826,560, the latter two patents being directed to the formation of gradient index waveguides.
The following problems have been encountered when practicing the method wherein the consolidated draw blank was formed with a longitudinal aperture caused by removal of the mandrel. Because of the dopants added to the core region of the blank to increase the refractive index thereof, the coefficient of expansion of that region is greater than that of the cladding region. Thus, when the blank cools after the consolidation process, the central or core region is put into a state of tension. The aperture in the center of the blank, which has remained after removal of the mandrel, presents a free surface in the region of high tensile force where breaks are readily initiated. Also, the rate of drawing a fiber from a preform having an aperture is limited by the rate at which the aperture will close during the drawing process. Moreover, the surface of an open preform aperture can become contaminated, especially during the high temperature fiber drawing operation.
A solution to the aforementioned problems is disclosed in U.S. Pat. No. 4,251,251 (Blankenship) wherein the preform aperture closes during consolidation due to the predetermined relative viscosities of the core and cladding materials. The Blankenship patent teaches that care must be taken to minimize damage done to the preform while removing the mandrel since such damage can result in the formation of seeds at the center of the resultant consolidated draw blank. Damage to the aperture surface is especially troublesome in the practice of that method since defects cannot be etched from that surface after consolidation, the aperture having been closed during that step. Damage to the aperture surface can be reduced by forming a soft parting layer of carbon soot on the surface of the mandrel and thereafter building up the preform with glass soot.
U.S. Pat. No. 4,298,365 teaches a method which further facilitates the closing of the soot preform aperture during consolidation. A thin stratum of very low viscosity glass soot is initially applied to the surface of a mandrel. First and second coatings of glass soot are then deposited on the surface of the thin stratum in accordance with the teachings of the aforementioned Blakenship patent. The mandrel is removed, and the soot preform is subjected to a high temperature whereby it is consolidated to form a dense glass blank. The glass surface tension and the relative viscosities of the inner and outer portions of the preform cause the aperture to close during the consolidation process. The thin stratum, which may include P.sub.2 O.sub.5 or B.sub.2 O.sub.3 smoothes over the damage caused by removing the mandrel and reduces or even eliminates seed formation at the axis of the resultant glass blank.
However, the use of P.sub.2 O.sub.5 or B.sub.2 O.sub.3 to facilitate hole closing is detrimental to achieving ultra low loss attenuation at long wavelengths. Fibers having cores that are free from P.sub.2 O.sub.5 or B.sub.2 O.sub.3, eg. fibers having cores consisting of GeO.sub.2 -doped silica, are preferred for low-loss operation in the infrared region of the spectrum. The aperture in a preform having a GeO.sub.2 -SiO.sub.2 core region does not close during consolidation.
In my copending application Ser. No. 402,056 entitled "Method of Making Glass Preform and Optical Fiber" filed on July 26, 1982, there is disclosed a process whereby a porous preform formed by the flame hydrolysis process is heated to form an elongated, consolidated glass preform having an aperture therethrough. One end of the aperture is closed, the aperture is evacuated and the other end is then closed. The resultant article can be handled or stored without risk of contaminating the aperture forming walls.
In accordance with one embodiment of the method of my copending application a layer of low expansion silica soot is initially formed on the mandrel so that, upon consolidation of the soot preform, the aperture forming surface is in compression, thus reducing the tendency for cracks to initiate at that surface. However, the surface of the pure silica layer adjacent the mandrel readily devitrified during consolidation. The devitrified surface portion of the silica layer had to be removed by etching to prevent the formation of seeds in the resultant fiber. Since one end of the preform aperture closed during consolidation, that end had to be severed to permit the acid wash to flow freely through the aperture in order to effectively remove the devitrified layer. The additional process steps of severing and etching increase the cost of producing optical fibers.